Astronomical Science Seeking for the Progenitors of Type Ia Supernovae Ferdinando Patat1 Type Ia supernovae are thought to be will allow us to put constraints on the the- Poonam Chandra 2,3 thermonuclear explosions of accreting ory of binary-star evolution. Roger Chevalier 2 white dwarfs that reach a critical mass Stephen Justham 4 limit. Despite their importance as cos- Having in mind why we want to do this, Philipp Podsiadlowski 4 mological distance indicators, the na- the next question is, as usual, how. A Christian Wolf 4 ture of their progenitors has remained discriminant between some of the pro- Avishay Gal-Yam 5 controversial. Observations carried posed scenarios would be the detection Luca Pasquini1 out by our team with VLT-UVES led us of circumstellar material (CSM). However, Ian Crawford 6 to the detection of circumstellar mate- notwithstanding the importance of the Paolo Mazzali 7, 8 rial in a normal Type Ia supernova. quest, all attempts at detecting direct sig- Adalbert Pauldrach 9 The expansion velocities, densities and natures of the material being transferred Ken’ichi Nomoto10 dimensions of the circumstellar en - to the accreting white dwarf in normal Stefano Benetti11 velope indicate that this material was SNe Ia were so far frustrated, and only Enrico Cappellaro11 ejected from the system prior to the upper limits to the mass-loss rate could Nancy Elias-Rosa 7, 12 explosion. The relatively low expansion be placed from optical, radio and UV/X- Wolfgang Hillebrandt 7 velocities appear to favour a progenitor ray emission. Claims of possible ejecta- Douglas Leonard13 system where a white dwarf accretes CSM interaction have been made for a Andrea Pastorello14 material from a companion star which is few normal objects, in which the pres- Alvio Renzini11 in the red-giant phase at the time of the ence of CSM is inferred by the detection Franco Sabbadin11 explosion. of high-velocity components in the SN Josh Simon 5 spectra. However, it must be noted that Massimo Turatto11 these features can be explained by a 3D The quest structure of the explosion and, therefore, circumstellar interaction is not necessarily 1 ESO Due to their enormous luminosities and a unique interpretation. Furthermore, no 2 Department of Astronomy, University of their homogeneity, Type Ia Supernovae velocity or density estimate is possible for Virginia, Charlottesville, USA (hereafter SN Ia) have been used in cos- the CSM material, even in the case that 3 Jansky Fellow, National Radio Astron- mology as reference beacons, with the the high-velocity components in the SN omy Observatory ambitious aim of tracing the evolution spectra are indeed the effects of ejecta- 4 Department of Astrophysics, University of the Universe (Riess et al. 1998; Perl- CSM interaction. of Oxford, United Kingdom mutter et al. 1999). Despite the progress 5 Astronomy Department, California Insti- made in this field, the nature of the pro- Two remarkable exceptions are repre- tute of Technology, Pasadena, USA genitor stars and the physics which gov- sented by the peculiar SNe 2002ic and 6 School of Earth Sciences, Birkbeck erns these powerful explosions are still SN 2005gj, which have shown extreme- College London, United Kingdom uncertain. In general, they are thought ly pronounced hydrogen emission lines, 7 Max-Planck-Institut für Astrophysik, to originate from a close binary system that have been interpreted as a sign of Garching, Germany (Whelan and Iben 1973), where a white strong ejecta-CSM interaction. However, 8 INAF – Osservatorio Astronomico, dwarf accretes material from a compan- the classification of these supernovae as Trieste, Italy ion until it approaches the Chandrasekhar SNe Ia has been questioned, and even if 9 Institut für Astronomie und Astrophysik limit and finally undergoes a thermonu- they were SN Ia, they must be rare and der Ludwig-Maximilians-Universität, clear explosion. This scenario is widely hence unlikely to account for normal Type Munich, Germany accepted, but the nature of both the ac- Ia explosions. As a matter of fact, the only 10 Department of Astronomy, University of creting and the donor star is not yet genuine detection may be represented Tokyo, Japan known, even though favourite configura- by the underluminous SN 2005ke, which 11 INAF – Osservatorio Astronomico, tions do exist (see Parthasarathy et al. has shown an unprecedented X-ray emis- Padova, Italy 2007 for a recent review). But why is it so sion, at a 3.6 S-level, accompanied by 12 Universidad de La Laguna, Tenerife, important to investigate the nature of the a large UV excess (Immler et al. 2006). Spain progenitor system? Besides the funda- These facts have been interpreted as the 13 Department of Astronomy, San Diego mental implications on the cosmological signature of a possible weak interaction State University, USA usage of SNe Ia, there are actually sev- between the SN ejecta and material lost 14 Astrophysics Research Centre, eral other reasons to bother (Livio 2000). by a companion star. Queen’s University Belfast, United First of all, galaxy evolution depends on Kingdom the radiation, kinetic energy and nucleo- All the channels explored so far to detect synthesis yields of these powerful events. CSM around Type Ia SN progenitors Secondly, the knowledge of the initial are based on the fact that sooner or later conditions of the exploding system is cru- the fast SN ejecta will crash into the cial for understanding the physics of slow-moving material lost by the system the explosion itself. Finally, identifying the in the pre-explosion phases in the form of progenitors and determining the SN rates stellar wind. This implicitly requires two 30 The Messenger 131 – March 2008 conditions to be fulfilled: a) there has to radiation field produced during the explo- ionisation, without the need for direct in- be interaction; and b) the amount of CSM sion. Therefore, is is reasonable to expect teraction. and its density must reach some thresh- variations in the physical conditions of old values in order to produce a detecta- the CSM, like dust evaporation and/or gas With this idea in mind, the experimental ble interaction. Therefore, methods based photoionisation. path was rather clearly traced: obtain on ejecta-CSM interaction will not be multi-epoch, high-resolution spectros- able to reveal this material if its amount is It was while investigating these effects copy of the next bright SN Ia and look for small and/or if it is placed rather far from that we saw a possible way of revealing absorption-line variability. the explosion site. But there is another low amounts of CSM material without possibility of revealing CSM, basically the need of having matter interaction. In because of the transient nature of the SN fact, since in SNe Ia the UV flux blue- SN 2006X in M100 event and its high luminosity. wards of 350 nm undergoes severe line blocking by heavy elements like Fe, Co, The first chance to test our idea came In fact, if the SN is surrounded by a dusty Ti and Cr, they are capable of ionising when SN 2006X was discovered in environment, the scattered light will add possible CSM only within a rather small the Virgo Cluster spiral galaxy M100 on with some delay to the SN signal (this is radius. Once the UV flux has significant- 4 February 2006 (Figure 1). A few days why this phenomenon is also known as ly decreased past the post-maximum later, the object was classified as a nor- a light echo), leaving certain signatures in phase, then, if the material has a suffi- mal Type Ia event occurring 1–2 weeks the observed light curves and spectra ciently high density, it can recombine, before maximum light and suffering sub- (see Patat 2005 for a review on this sub- producing time-variable absorption fea- stantial extinction. Prompt Very Large ject). These effects are expected to be tures. Of course, if the material where Array (VLA) observations have shown no dependent on the distance of the scatter- these features arise is reached by the radio source at the SN position, estab- ing material from the SN itself. More pre- fast-moving ejecta, it will be shocked and lishing one of the deepest and earliest cisely, if the dust is contained in a distant ionised, causing the disappearance of limits for radio emission from a Type Ia, cloud (like for example in an interven- such absorptions. and implying a mass-loss rate of less ing spiral arm of the host galaxy), the late than a few 10–8 solar masses per year (for time epochs of the observed SN evolu- Among all possible inter/circumstellar ab- a low wind velocity of 10 km s–1). The SN tion will be completely dominated by the sorption lines, the ubiquitous sodium was not visible in the 0.2–10 keV X-rays light echo, as in the well-known cases D lines (589.0 and 589.6 nm) are the best band down to the SWIFT satellite detec- of SNe 1991T and 1998bu. On the con- candidates for this kind of study. In fact, tion limit. All of this made of SN 2006X a trary, if the scattering material is confined besides falling in an almost telluric ab- perfect candidate to verify our idea. within a small region surrounding the SN, sorption-free spectral region, they are the effects although present at all epochs, produced by a very strong transition, and An ESO Director General Discretionary are subtle and can be confused with in- hence detectable for rather small gas Time proposal was submitted on 15 Feb- trinsic SN properties (Patat et al. 2006). column densities. In addition, the ionisa- ruary and approved immediately after- tion potential of Na I is low (5.1 eV), and wards.